Wireless cellular communications around the world are moving toward CDMA systems in the third generation approaches. A CDMA system has the most efficient usage of the limited radio spectrum. It has been proved that a CDMA system ideally provides a large channel capacity gain over other accesses methods such as Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA). In general, a CDMA system has to use a RAKE receiver to combine the received signal energy form different paths to combat the effect of multipath fading.
A RAKE receiver implements a form of path diversity by gathering the signal energy from different paths and by optimally combing all the multipath signals together. The path diversity function provides a robust communication channel so that when one path fades, communication is still possible through a non-fading path. A CDMA RAKE receiver detects the multipath signals using either a matched filter method or a correlation method.
Conventionally, a RAKE receiver in a CDMA system uses a spreading code matched filter to despread a multiple access code and a transversal filter for channel impulse response matching. The spreading code matched filter can be implemented at IF band using a SAW filter or at baseband using a digital matched filter. After code despreading, a transversal filter which is implemented at baseband is used to combine the received signal energy form different paths. The drawback of the SAW filter approach is that a SAW filter cannot be easily integrated with the baseband transversal filter in an IC. As IC technology progresses rapidly, the digital spreading code matched filter approach is a preferred choice. Although the current IC technology can provide large computational power, it is still hard to implement a transversal filter based RAKE receiver in IC, especially when the length of the multiple access code is large.
An alternative method to implement a RAKE receiver is to use a bank of correlators. Each correlator is used to detect a received signal path separately. The number of the correlators in the correlator bank is typically three or four. Therefore, this RAKE receiver structure needs to search for three or four stronger paths in the received signal.
Both the above two RAKE receiver implementations need a sounding receiver to estimate the multipath channel impulse response. A RAKE receiver needs to know the delay time, the carrier phase shift, and the strength of the main paths. Moreover, the correlator bank implementation needs extra computations to select the main paths.
One method to increase the CDMA system capacity is to use a parallel interference cancellation (PIC), for example, as described in Dariush Divsalar and Marvin K. Simon, "Improved CDMA performance using parallel interference cancellation," IEEE MILCOM, pp. 911-917, October 1994. Although the CDMA system capacity as this paper described can be increased, their receiver is applicable to a Additive White Gaussian Noise (AWGN) channel only and is unsuitable for a multipath fading channel. Other PIC methods for a multipath fading channel have also been proposed, such as in Matti Latva-aho, Markku Juntti and Markku Heikkila, "Parallel Interference Cancellation Receiver for DS-CDMA Systems in Fading Channels," IEEE PIMRC, pp. 559-564, September 1997. However, these methods adapt a time domain signal processing approach to cancel the multiple access interference and therefore, become complicated.